John O'Farrell, Labour's parliamentary candidate for Eastleigh: ‘“One nation” chimes with exactly the sort of Labour party I want us to be. So I put on a suit and I pin on a red rosette.’ Photograph: Chris Ison/PA

I'm not quite sure how this happened. A week ago I was peacefully writing away in the London Library, thinking February was looking a bit quiet. Then the Eastleigh byelection was called, with an indecent three weeks' campaigning time till polling day. A fellow Labour party member suggested I should be the candidate. I had not been thinking of standing again any time soon, but I saw her logic. So many politics students have been forced to read my book about being a Labour activist in opposition, maybe they'll be under the illusion that canvassing in the rain is all one big laugh? Plus, I had a few press contacts; I could definitely get on to the Breakfast Show for BBC Radio Solent.

So I declared an interest on Twitter and was amazed with the avalanche of goodwill towards the idea.

Suddenly, I feel really lucky to have an outlet for the profound sense of outrage I feel about this coalition government. This is the most important and interesting byelection for years, with the bizarre situation of Liberals and Tories fighting each other for the right to vote for exactly the same terrible policies. How could I not want to get in and help Labour stop both of them?

The well-oiled Labour byelection machine is something to behold, and organisers here are keen to use me as effectively as possible: meeting voters, writing leaflets, and suggesting that my anti-apartheid badge might be a bit out of date now. I do my best to follow the official Labour script, though for people of my age who remember the Funkadelic hit, it's hard to say "one nation", without then singing "under a groove … " When I'm talking to voters, I keep doing that Miranda thing of telling myself "Don't sing!".

In fact, "one nation" chimes with exactly the sort of Labour party I want us to be. So I put on a suit and I pin on a red rosette. (An American friend saw it and said: "What the hell is that? Looks like you won a prize for baking a really great pie!")

You can't just be your normal everyday self in democratic politics. You have to put on clothes you wouldn't normally wear – metaphorically and literally. Lots of us on the left like the idea of not conforming, of each doing our own thing. Another phrase for this is "losing elections". Is that why the left has not built enough on past victories, because we are more comfortable protesting than governing? Just by having had turns in power the Labour party is seen by many as part of the establishment, and so purists keep themselves untainted by sneering at any party that could win and actually pass legislation.

But I am in Labour because it is the largest party of the left that is ever likely to be elected to power. If the Socialist Workers party or Respect or People's Front of Judea ever grew large enough to challenge for a parliamentary majority, they too would have to face all the same difficult policy choices and compromises to persuade moderate doubters to come on board. "Hmmm, red or purple catsuit?" George Galloway would ask his style consultants; "What are the focus groups saying in the Ukip/Respect marginal?"

The compromises were massive in the past decade or so, understandably more than many could bear. US politician Mario Cuomo said: "You campaign in poetry, you govern in prose." I'm afraid it's worse than that. We'll probably govern in Tweet-speak. C U @ #Eastleigh #LabourDoorstep lol ;-)

But if you consider yourself to be anywhere on the left, you either work to secure a Labour government, or you are just a protester (which is fine if that's your thing, but don't expect us to give up trying to win actual power).

Now we have seen the alternative in action, and I can feel the support for Labour flooding back.